Unless the print is greatly over-exposed or reversed, the image appears in white lines on a blue background as soon as washing is com menced. Water which contains large quantities of calcium salts causes the formation of a deposit of yellowish basic iron salts on the image, and at the same time the image is weakened by their alkalinity, the weakening becoming more pronounced if the washing is prolonged. These defects may be remedied by carrying out the final washing in water which has been acidified with a little hydrochloric acid, which lightens the whites and strengthens the blues of the image. This acid must on no account be added to the first wash water, as it brings about formation of a general blue fog. Hydrochloric acid treatment and the treatments mentioned below cannot be carried out in zinc dishes, as the metal is very rapidly attacked.
Over-exposed prints, in which the lines are more or less strongly blued, and prints which have undergone reversal, in which the lines are darker than the ground, may be restored to a normal condition by treating with an oxidizing solution (a very weak solution of hydrogen peroxide, eau de Javelle, potassium biclirornate, persulphate, chlorate, etc., neutral or slightly acidified). Such treatment should be deferred until after the second washing at least, and should be followed by washing in several changes of water.
If no drying apparatus is available, the prints are hung on lines of cord or string, or are laid flat on frames covered with canvas or wire netting. If the frames are fitted with the netting, the sheets should be dried face upwards, as contact with the metal destroys the image.
Blocking-out is done with a paint-brush, using prussian blue water-colour. White lines are added with a pen dipped in a 20 per cent solution of neutral potassium oxalate, thickened with a little gum arabic (a 5 per cent solution of oxalic acid can also be used, but it causes rapid corro sion of the nibs). Parts retouched in this way should be washed or at least briefly rinsed (with the help of a sponge), otherwise the blue colour is liable to re-appear after a time.
As perspiration is always alkaline, one should avoid touching the dry prints with moist hands. The hands can be conveniently dried with talc powder if this is the case.
Ferro-prussiate prints fade if exposed to bright light for a long time. They can be restored by keeping in the dark, especially if the atmosphere is damp.
A large number of methods have been sug gested by means of which ferro-prussiate paper can be toned in various colours. The results obtained are usually disappointing.
623. True-to-scale Process. A process origin ated by F. and J. Dorel (Iwo) allows a relatively large number of copies (up to about 20) to be obtained from a ferro-prussiate print, the latter being used immediately after its removal from the printing frame, and without washing. The lines are produced on a white base in greasy inks of any desired colour, and the prints are free from the distortions and deformations which occur when a paper is wetted and dried.
For the purpose a jelly paste is used (con sisting of a strong solution of gelatine to which some glycerine, ferrous sulphate, ox-gall, an antiseptic, and a yellow or white pigment have been added), which, after it has been melted on a water-bath, is run in a layer about in. thick on to a wooden slab covered with thin, well-stretched zinc which has been roughened with glass-paper to facilitate adherence. When the jelly is properly set, it is ready for printing.
The prints to be used for the purpose should be made on slow ferro-prussiate paper sensitized with a comparatively concentrated mixture in which the ferric salt should be in excess, giving clean and vigorous images, while over-exposure, and especially reversal of the image, should be avoided.
The print from the printing frame is carefully laid face downwards on the surface of the gelatine mixture with one application, avoiding air-bubbles and creases ; good contact is assured by gently rubbing with a large pad of soft rags. After remaining for about a minute in contact, the paper is lifted off, and the gelatine quickly inked over with typographic or copper-plate ink, spreading it with a gelatine roller of the type used in letterpress printing. The ink adheres only to the parts corresponding with the lines of the image. The markings caused by creases, breaks, or stains on the tracing can be cleaned off with a damp sponge if necessary. Printing should be done on a well-sized paper, pressure being applied with a pad or a roller when the paper has been applied to the inked surface. After the paper has been removed, the ink can be dried with powdered talc if the print is re quired for immediate use.
Normally, the gelatine is kept moist by the glycerine, and will not take the greasy ink. When the ferro-prussiate paper is applied, the excess of ferricyanide in the parts protected by the lines on the tracing is absorbed by the damp gelatine with which it is in contact, and, reacting with the ferrous salt, forms a blue line. The prussian blue, formed in this way, brings about tanning of the gelatine, which is then able to retain the greasy ink.